PHP中的MySQL查询不好？

Question

我无法正确触发此查询。我从Apache获得了500个错误，并提供了详细的错误消息：

  

“类mysqli_result的对象无法转换为var / www / website / sqlhandler.php中的字符串”

我还有其他查询工作正常，所以我不确定这里的问题是什么。已正确设置teacherid会话变量，并且可以使用此函数。

 classname = $_REQUEST['classname'];
 classlength = $_REQUEST['classlength'];
// Create connection
    $conn = new mysqli($servername, $username, $password, $dbname);
    // Check connection
    if ($conn->connect_error) {
        die("Connection failed: " . $conn->connect_error);
    } 



$result = mysql_query("INSERT INTO Classes (classLength, teacherID, className) VALUES ('$classlength' ',' '".$_SESSION['teacherid']." ',' '$classname');");
       if ($result)
       {
           $_SESSION['classname'] = $classname;
           $_SESSION['classlength'] = $classlength;
           echo 'success';
       }
    }